To be able to describe the invention and also show the technical problems in connection with rolling of strip that the invention is capable of solving, by way of example a very brief description will first be given of the most important parts of a finishing mill in which the invention may be used.
Such a mill normally consists of up to 6 or 7 mill stands positioned one after the other with a subsequent cooling distance of up to 80-100 m and a coil for rolling up the rolled and cooled strip.
Between the mill stand, so-called loopers are positioned with a roller over which the strip runs. The looper is rotatably eccentrically journalled in relation to the centre of the roller. The rolling is controlled with respect to keeping the angle between a line in the horizontal plane and the line between the centre of rotation of the looper and the centre of the roller constant and equal for all the loopers, and with respect to a speed control. As the strip passes through the mill stand, the speed of the strip increases. For this reason, there is also a superordinate control which adjusts the speed for the rolls of each mill stand so that the angle of the loopers can be kept constant.
The roller of the looper is used as a measuring roller and is therefore divided into a number of sub-rollers which may be loaded with a force which can be varied within quite large limits depending on the winding angle of the strip, the desired accuracy of the flatness profile, the strip thickness in question, the temperature, the material composition, etc. A total force on the measuring roller of 30-50 kN and more may very well exist.
There are several factors which may lead to the flatness of the strip not being the desired one. There are also some different methods to influence the strip flatness so as to attain the desired flatness. To be able to perform some form of control of the strip flatness, it is necessary in some way to determine the flatness of the strip or the strip profile across the strip.
One of the main requirements on a good strip mill is that the coiled strip shows good flatness. To achieve this, considerable knowledge of the process is normally required. A flat, coiled strip need not always mean that the strip as it leaves the last mill stan shall have a straight flatness profile. As a matter of fact, in order to obtain a flat strip during the coiling, the optimum profile of the strip as it leaves the last mill stand is dependent on the cooling process of the strip, etc.
The profile that the strip has when it arrives at the last mill stand is substantially dependent on the original flatness of the strip and on the mutual parallelism and curvature of the rollers mounted in the mill stand. If these rollers are not correctly adjusted, the strip may exhibit buckles as a result of the strips--according to the technical terminology --becoming "long edge" or "long middle". Thus, the strip may have a tendencey to become longer at the edges and at the centre of the strip, respectively.
As stated above, the rolling of the strip may entail problems in obtaining the desired properties of the end product. If the current strip flatness is available, for example between the last two mill stands, it will be possible to influence the flatness by adjusting the rollers mounted in the mill stands. The desire to have knowledge of the strip profile at or near the last mill stands in a strip mill has existed for a long time and various methods and means have been developed--with more or less successfull results.
A rough estimate of the unflatness of cold-rolled or hot-rolled strip is described in U.S. Pat. No. 4,512,170. Here the measuring roller is not divided into measuring zones, but it is stated that with knowledge of the current forces, the deflection or bending moment on the shaft of the measuring roller in the vicinity of its ends, and by comparison with theoretically predicted values of unflatness for these parameters, a measure of the current strip profile can be obtained. The accuracy of such a method cannot, with all necessity, be particarly great.
U.S. Pat. No. 4,116,029 discloses a measuring roller for measuring the flatness of rolled strip, the measuring roller being divided into a number of measuring zones, each having a roller section supported by two side members. By measuring the force to which each side member is subjected, a measure of the flatness of the strip can thus be obtained. The embodiment in this patent which may be of a certain interest for the invention, as showing the prior art, is shown in FIG. 3 of the patent. In this embodiment the measuring element is mounted on an arm which is hinged to the side members and is movable in three hinges. In such a design, there must be a considerable risk of play and friction in the hinges. This in turn leads to poor positional tolerance of the rollers and to hysteresis of the transducer signals. Since each roller section, as well as other known measuring rollers divided into roller sections, have two side members, the gap which occurs between the rollers may become quite significant, which may lead to a risk of marking of the strip. There is also a considerable risk of the gap being filled with oxide scales from the strip which may give rise to an uncontrolled coupling between adjacent measuring rollers.
An extensive description of known principles for flatness measurement is given in and article entitled "New developments improve hot strip shape: Shapemeter-Looper and Shape Actimeter" published in "Iron and Steel Engineer", August 1986, pages 48-56. FIG. 9 of this article is a reproduction of the embodiment described above.
As belonging to the state of the art, however without relevance to the invention, can also be mentioned equipment for non-contact determination of the flatness of a strip with the aid of optical systems and also with the aid of acoustic waves.